UC advisor supports burning alternative

May 03, 2003

Since humans first shifted from gathering food to growing it, agricultural burning has been part of the process. Today, some 9,000 years later, a UC Cooperative Extension farm advisor has found a viable alternative to a practice that is contributing to the San Joaquin Valley's already daunting air pollution problem.

Madera County farm advisor Brent Holtz' research focuses on chipping, shredding and spreading almond tree prunings, which typically are raked into brush piles each fall and ignited. When Holtz sees such a burn, he shudders to think about the potential soil benefits that are going up in smoke.

Holtz' personal experience and research show that chipped and shredded prunings left in the orchard, where they break down in the soil, create a healthy soil micro-environment, add nutrients and improve soil tilth.

Growers of peaches, nectarines, plums, pistachios and other crops are somewhat more likely to implement such an alternative to ag burning. However, almond and walnut growers have been slow to adopt this approach because they fear their crop, which is shaken to the ground and then swept up at harvest time, would lose value if contaminated by wood chips.

Cost as factor

Cost is another obstacle. While it is virtually free to light a bonfire, shredding requires renting or purchasing special equipment. But Holtz believes tightening regulations related to ag burning, combined with the soil benefits, make the cost well worth it.

The San Joaquin Valley Air Pollution Control District is planning to revamp its ag burning regulations in early 2003, tossing out the toggling “ag burn” and “no burn” days that are a familiar part of valley weather forecasts. Valley Air District spokeswoman Josette Merced Bello said the new ag smoke management system will divide the valley into 93 small zones.

“Everyday will be a burn day,” Bello said. “Each day we will determine how much acreage and what material can be burned, if any, for each of the 93 zones. Farmers who have materials to burn will call us to find out whether it would be permissible to burn on a certain day.”

Avoiding ag burning altogether returns control to farmers.

Holtz' interest in ag burning alternatives goes back much further than his employment with UC Cooperative Extension. A farm advisor since 1994, Holtz, who holds a doctorate degree in plant pathology, grew up on a family almond farm near Modesto, Calif. His father began using a small garden shredder around the house, in which he shredded expired annuals and twigs, branches and leaves from their ornamental trees and shrubs. They spread the shredded material over soil in their landscape and realized, after a few years, that the soil seemed richer and easier to work.

In 1988, the Holtzes rented a commercial wood chipper and took their experiment to the farm. The father-son team found that wood chips incorporated themselves with the soil and began breaking down well before harvest season. They observed other positive indicators of soil health. “I counted hundreds of mushrooms on our orchard floor, compared to none on neighbors' farms where they were burning their almond prunings,” Holtz said.

He was sold, but as a researcher himself he knew the anecdotal evidence wouldn't stand in the scientific community. Holtz began replicated research trials to demonstrate the benefits.

To begin with, he grew almond trees in pots that contained potting soil and different amounts of 1/2-inch-sized wood chips. The plants were never fertilized.

The first year, trees growing with wood chips were stunted. But by the second year, the trees with and without wood chips were growing about even. By the third year, the trees with the wood chips were growing stronger and had significantly more nitrogen.

“As the wood chips break down, at first they absorb nitrogen. Later in the decomposition process, nitrogen is released,” Holtz said. “Additional nitrogen may also be provided by bacteria decomposing the wood chips. That's pretty exciting.”

The trees were grown without added fertilizer for comparison purposes. In a commercial orchard, a farmer might add nitrogen to offset the tendency for decomposition to make some nitrogen unavailable to the trees at first.

“We have been wood chipping in our family almond orchard for 14 years and we have never seen our trees get nitrogen deficient,” Holtz said.

In a second experiment, conducted in association with UC Kearney Ag Center nemotologist Mike McKenry and recently presented to the scientific community at a meeting in Toronto, Canada, the scientists found that the wood chips appear to favor nematodes that do not damage trees.

“The soil with wood chips is building up high populations of free-living nematodes and reducing numbers of parasitic nematodes that attack trees' roots,” Holtz said. “There are more good guys living in soil that used to be inhabited by the bad guys. Nematodes are a big problem. Wood chips seem to help farmers out. Why burn them when your soils can use the organic matter?”